title: Diurnal fluctuations in seawater pH influence the response of a calcifying macroalga to ocean acidification.
abstract: Coastal ecosystems that are characterized by kelp forests encounter daily pH fluctuations, driven by photosynthesis and respiration, which are larger than pH changes owing to ocean acidification (OA) projected for surface ocean waters by 2100. We investigated whether mimicry of biologically mediated diurnal shifts in pH-based for the first time on pH time-series measurements within a kelp forest-would offset or amplify the negative effects of OA on calcifiers. In a 40-day laboratory experiment, the calcifying coralline macroalga, Arthrocardia corymbosa, was exposed to two mean pH treatments (8.05 or 7.65). For each mean, two experimental pH manipulations were applied. In one treatment, pH was held constant. In the second treatment, pH was manipulated around the mean (as a step-function), 0.4 pH units higher during daylight and 0.4 units lower during darkness to approximate diurnal fluctuations in a kelp forest. In all cases, growth rates were lower at a reduced mean pH, and fluctuations in pH acted additively to further reduce growth. Photosynthesis, recruitment and elemental composition did not change with pH, but δ(13)C increased at lower mean pH. Including environmental heterogeneity in experimental design will assist with a more accurate assessment of the responses of calcifiers to OA.
authors.affiliation: Department of Botany, University of Otago, , PO Box 56, Dunedin, New Zealand, Department of Marine Sciences, University of Otago, , PO Box 56, Dunedin, New Zealand, Department of Chemistry, University of Otago, , PO Box 56, Dunedin, New Zealand, National Institute for Water and Atmospheric Research Ltd (NIWA), Centre for Physical and Chemical Oceanography, University of Otago, , PO Box 56, Dunedin, New Zealand, School of Chemistry and Biochemistry, University of Clark, , 950 Main Street, Worcester, MA, USA, School of Biological Sciences, University of Waikato, , Private Bag 3105, Hamilton, New Zealand.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Cornwall
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Hepburn
authors.forename: Christina M
authors.initials: CM
authors.lastname: McGraw
authors.forename: Kim I
authors.initials: KI
authors.lastname: Currie
authors.forename: Conrad A
authors.initials: CA
authors.lastname: Pilditch
authors.forename: Keith A
authors.initials: KA
authors.lastname: Hunter
authors.forename: Philip W
authors.initials: PW
authors.lastname: Boyd
authors.forename: Catriona L
authors.initials: CL
authors.lastname: Hurd
mesh_terms: D002118:Calcium / Q000378:metabolism; D002254:Carbonates / Q000378:metabolism; D057231:Climate Change; D006863:Hydrogen-Ion Concentration; D009520:New Zealand; D010101:Oxygen Consumption; D010788:Photosynthesis; D000461:Rhodophyta / Q000254:growth & development* / Q000378:metabolism*; D012623:Seawater / Q000737:chemistry*; D013997:Time Factors
chemical_list: D002254:Carbonates; D002118:Calcium
keywords: coralline macroalgae; environmental heterogeneity; natural pH fluctuations; ocean acidification; temperate rocky reefs